Process for preparing white lead



Patented Apr. 30, 1940 UNITED STATES FFICE ff PROCESS FOR PREPARING W HITE LEAD Edward D. Turnbull, Clarks Summit, -Pa., as-

signor to The Glidden Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application January 19, 1938,

Serial No. 185,653

5 Claims.

of passing carbon dioxide into basic lead acetate solution, has always been of poor quality, from the point of view of hiding power and tinting 15 strength. More recently, I have developed a process of making white lead by controlling the reaction conditions, as disclosed in my co-pending application Serial No. 83,275, filed June 3, 1936, now Patent No. 2,115,090, granted April 26, 1938, 20 whereby a satisfactory pigment is obtained by this method. The control results in the production of a pigment while the solution is on the acid side of the neutral point, at a pH of below 6.0; the

pigment has an acid pH. Other white leads made 25 with the final pH of the solution on the acid sides show similar acidity.

When used in the ceramic industry, in making glazes, it has been discovered that these acid white leads produce glaze slips of much heavier 30 consistency than produced by white leads which have their pH above 7.0 made by the electrical and other processes. An attempt was made to obtain improved viscosities by treating the pulp with alkali before filtration, thus increasing the 35 pH of the pigment and reducing the viscosity of the slip. This attempt, however, gave unsatisfactory results, in that the desired viscosities are not obtained without the use of detrimental quantities of alkali.

L Further investigation of the problem resulted in the discovery that the acid-precipitated white leads contain small quantities of lead acetate, either absorbed in the pigment or adsorbed on the surface of the particles, and that this lead ,5 acetate is responsible for the high viscosity imparted to glaze slips. In View of the character of the union between the lead acetate and the pigment, reaction does not occur rapidly in the slurry of the pulp, between the added alkali and 0 the adherent lead acetate, so that mere adjustment of the alkalinity does not produce the desirable low viscosity with sufficiently small quantitles of alkali. I have discovered that if an alkali carbonate be added to the filter cake, and.

the mixture dried while subject to agitation, the

carbonate is adsorbed onto the pigment, or absorbedby it, in the same manner as the lead acetate, and that the surface acidity of the pigment is destroyed by reaction between the lead acetate and the alkali carbonate. The resultant pigment gives desirable low viscosity ceramic glaze slips.

As a specific example of my invention, I precipitated white lead from a basic lead acetate solution in the conventional manner, the final pH of the solution being 5.9. The precipitate was treated in a Dorr thickener to prepare a filterable slurry; this was put through an Oliver suction filter, and a batch of filter cake was, ob tained having 300 pounds of white lead together with 517 pounds of water. The batch was charged into a vacuum drier equipped with an agitator, and 1 pounds of anhydrous sodium carbonate were added. The drier was allowed to run at 180 F. under a vacuum of 20 inches for 15 hours; the dried pigment which came out was of a character which produced satisfactory low viscosity ceramic glaze slips.

The use of a vacuum drier in this connection is of considerable importance. Working with either treated or untreated white lead, superior results are obtained, as to viscosity of slip, by the use of a vacuum drier, the glaze slips of air dried mate-- rial being considerably more viscous than vacuum dried material of the same pH Agitation during the drying is also important, as, improved results areobtained by its use.

Other white leads precipitated from lead ace tate solutions with a final acid concentration are affected similarly by this treatment, as are white leads made by other processes which yield a final acidity.

The amount of alkaline carbonate added should be kept at a low figure, as, excess alkali is undesirable. I find about .4 or .5% to be a good figure. in ordinary practice; amounts over 1.0% should be avoided, as the alkali is then present in too substantial concentration.

In the claims, the term absorbed means absorbed in the interstices, or adsorbed on the surface.

I claim:

1. The process of treating basic carbonate white lead containing absorbed or adsorbed lead acetate on the particles of basic carbonate white lead, which comprises incorporating a minor amount of anhydrous alkali carbonate sufiicient to neutralize the lead acetate into a water wet filter cake of the white lead, and drying the filter cake with agitation, whereby the lead acetate is converted into lead carbonate.

2. The process. of treating basic carbonate white lead containing absorbed or adsorbed lead acetate on the particles of basic carbonate white lead which comprises incorporating a minor amount of anhydrous alkali carbonate sufiicient to neutralize the lead acetate into a water wet filter cake of the white lead, and vacuum drying the filter cake with agitation whereby the lead acetate is converted into lead carbonate.

3. The process of treating basic carbonate cake of the white lead in amount up to 1% by white lead containing absorbed or adsorbed lead acetate on the particles of the basic carbonate white lead, which comprises incorporating an anhydrous alkali carbonate into'a water wet filter cake of the whitelead in amount up to 1% by weight of the white lead sufficient to neutralize the lead acetate, and drying the filter cake withagitation whereby the lead acetate is converted into lead carbonate.

4. The process of claim 1 in which the alkali carbonate is sodium carbonate.

5. The process of treating basic carbonate white lead containing absorbed or adsorbed lead acetate on the particles of the basic carbonate white lead, which comprises incorporating an anhydrous alkali carbonate into a water wet filter weight of the white lead sufiicient to neutralize the lead acetate and vacuum drying the filter cake with agitation whereby the lead acetate is converted into lead carbonate.

' EDWARD D. TURNBULL. 

